Every body of rock, no matter how strong, has a point at which it will deform by bending or breaking. Deformation (de = out, forma = form) is a general term for the changes in the shape or position of a rock body in response to differential stress. Most crustal deformation occurs along plate boundaries. Plate motions and plate interactions along their margins generate the tectonic forces that cause rock to deform.
The basic geologic features that form as a result of the forces generated by the interactions of tectonic plates are called rock structures, or geologic structures (Figure 1). Rock structures include folds (wave-like undulations), faults (fractures along which one rock body slides past another), joints (cracks), and small-scale structures associated with metamorphic rocks such as foliation and rock cleavage.
The Force That Deforms Rocks From everyday experience, you know that if a door is stuck, you must expend energy, called force, to open it. Geologists use the term stress to describe the forces that deform rocks. Whenever the stresses acting on a rock body exceed its strength, the rock will deform—usually by one or more of the following processes: folding, flowing, fracturing, or faulting. The magnitude of stress is not simply a function of the amount of force applied but also relates to the area on which the force acts. For example, if you are walking barefoot on a hard surface, the force (weight) of your body is distributed across your entire foot, so the stress acting on any one point of your foot is low. However, if you step on a small pointed rock (ouch!), the stress concentration at that point on your foot will be high. Thus, you can think of stress as a measure of how much force is applied over a particular area.
When stress is applied uniformly in all directions, it is called confining pressure (Figure 2A). Confining pressure causes the spaces between mineral grains to close, producing more compact rocks that have greater densities. Confining pressure does not, however, change the shape or orientation of a rock body. By contrast, when stress is applied unequally in different directions, it is termed differential stress. We will consider three types of differential stress:
- Compressional stress. Differential stress that squeezes a rock mass as if placed in a vise is known as compressional stress (com = together, premere = to press) (Figure 2B). Compressional stresses are most often associated with convergent plate boundaries. When plates collide, Earth’s crust is generally shortened horizontally and thickened vertically. Over millions of years, this deformation produces mountainous terrains.
- Tensional stress. Differential stress that pulls apart or elongates rock bodies is known as tensional stress (tendere = to stretch) (Figure 2C). Along divergent plate boundaries where plates are moving apart, tensional stresses stretch and lengthen rock bodies. For example, in the Basin and Range Province in the western United States, tensional forces have stretched and fractured the crust by as much as twice its original width.
- Shear stress. Differential stress can also cause rock to shear, which involves the movement of one part of a rock body past another (Figure 2D). Shear is similar to the slippage that occurs between individual playing cards when the top of the deck is moved relative to the bottom (Figure 3). Smallscale deformation of rocks by shear stresses occurs along closely spaced parallel surfaces of weakness, such as foliation surfaces and microscopic fractures, where slippage changes the shape of rocks. By contrast, at transform fault boundaries, such as the San Andreas Fault, shear stress causes large segments of Earth’s crust to slip horizontally past one another.
A Change in Shape Caused by Stress When flat-lying sedimentary layers are uplifted and tilted, their orientations change, but their shapes are often retained. This type of deformation results in a change in orientation (tilting) of the once flat-lying sedimentary layers and is called rotation. Deformation may also cause a rock body to change location—a process called displacement. For example, displacement occurs during faulting when the rocks on one side of the fault move relative to the rocks on the opposite side. This type of deformation changes the location of a rock body but does not substantially change its shape or orientation. Differential stress can also change the shape of a rock body, a type of deformation that geologists call strain. Like the circle shown in Figure 3, strained bodies lose their original configuration during deformation. Strain is illustrated by the deformed trilobite fossil show in Figure 4. When we compare the deformed specimen to a trilobite fossil of the same species that has not been deformed, the type of strain that occurred to the surrounding rock body can be determined. Stress is the force that acts to deform rock bodies, while strain is the resulting distortion, or change in the shape of the rock body.
E. J. Tarbuck, F.K. Lutgens, Illustrated by D. Tasa